Paints are widely used in households for application to both interior and exterior surfaces. For interior surfaces, particularly in kitchens and living areas the resistance of a paint film to domestic stains is an important property. This is especially the case for trim and wall paints that are frequently exposed to domestic stains.
Domestic staining materials include food, oil or grease and beverages such as coffee and red wine. For semi and low gloss interior paints, properties such as mar resistance and wet and dry scrub properties are also important. Good performance for these properties enables the stained surface to be cleaned by scrubbing without causing the treated area to “gloss up” and exhibit a gloss that is different from the surrounding area. However, it is clearly preferable that the paint be not very susceptible to staining materials as well as being able to be effectively cleaned.
Painted surfaces are also susceptible to staining from the substrate, especially in the case of timber substrates containing tannins. The tannin material can “print through” the paint film, discolouring the paint after it has dried. This problem is particularly evident with water-borne latex paints. Various factors can effect this substrate staining and special water-borne latex paints have been developed to overcome this problem. However, in this invention we are not concerned with this type of staining but staining on the outermost surface of the paint. Paints that perform well for substrate staining do not necessarily exhibit good resistance to domestic stains and vice-versa.
Conventional organic solvent based paints that have alkyd resins as binders and which form tightly crosslinked paint films generally exhibit excellent stain resistance properties. It is believed this is due to the low solubility of the staining material in the paint film. However, these paints are losing favour because of the combined adverse effects of volatilised organic solvents on the environment and inconvenience for the applicator from long recoat times and clean up requiring the use of organic solvents. Water-borne versions of alkyds suffer from yellowing problems, particularly in kitchen areas and have not yet met the needs of the market. Conventional water based or latex paints in which the paint film is formed by the coalescence of vinyl or acrylic thermoplastic polymer particles are widely used but hitherto have not exhibited excellent stain resistance together with the required balance of other paint properties, including hardness, film coalescence, pH viscosity stability, and tint strength.
It is essential for adequate mechanical film properties of conventional water based paints that there is good polymer particle coalescence over the range of temperatures encountered in ordinary use. Coalescence is directly related to the hardness or glass transition temperature (Tg) of the polymer particles. Low Tg polymers allow coalescence at lower temperatures than for higher Tg polymers. The hardness of a paint film influences properties such as scuff and abrasion resistance. Maximising film hardness is important especially for paints in kitchens and living areas. Hardness is usually increased by increasing the Tg of the polymer particles. However, enhancing the hardness by increasing the Tg of the polymer in the particle leads to poorer coalescence. In practice the compromise between these properties is overcome by using a volatile temporary organic plasticiser that allows the use of higher Tg polymer particles than otherwise would be able to be used. However, such formulations suffer from the disadvantage of poor early film hardness. This occurs as the volatile plasticiser may take some days before completely volatilising and being removed from the paint film. The use of volatile plasticisers is also not favoured in some applications as it leads to the release into the atmosphere of organic solvents. For trim paints it is desirable that they have film hardness greater than conventional water-borne latex paints intended for broad wall application.
Viscosity stability is an important property for commercial paint formulations. Paint products are commercially manufactured within permissible tolerances to predetermined specifications It is important that satisfactory paint characteristics are maintained across the allowable range in the specification. With regard to aqueous latex paints, pH is a manufacturing variable that may be in a range of, for example, 8.5 to 9.5. Viscosity is a paint property that may vary with pH change and for such paints it is important that viscosity at both low and high shear rates is within acceptable limits throughout the pH specification range. Viscosity at low shear rates tends to influence flow and levelling properties. Viscosity at high shear rates affects ease of brushing. A further aspect of viscosity stability is the effect of storage on this property. It is obviously desirable that viscosity does not significantly change with time given the possible long shelf life of commercial products.
The tint strength of a paint is an important property as it indicates whether a paint can be tinted to a particular colour using a standard tinting formula. It is a requirement that paints across a range of product types have similar tint strength so that common tinting formulae may be used across that range. The tint strength is a measure of the amount of coloured tinter that must be added to a white base paint to give a particular colour. If the base paint requires a smaller amount of the tinter it is described as being of low tint strength. Paint with low tint strength may be reformulated with higher levels of titanium dioxide pigment but this can add significantly to formulation costs. Conversely if the tint strength of a base paint is high it would be possible for it to be reformulated with lower levels of titanium dioxide pigment to give the required tint strength at a lower formulation cost. For such a reformulation to be acceptable opacity would be required to be maintained. Accordingly, it is desirable that the tint strength is close to or equal to other products across the range without having to reformulate the product with high levels of titanium dioxide pigment. In addition, tint strength must be stable over time because of the possible long shelf life of paints.
The binders used for conventional water based paints include a wide range of materials. A recent review article entitled “Polymers for Water-Based Coatings—A Systematic Overview” by J. C. Padget in Journal of Coatings Technology, Vol 66. No. 839, December 1994 at pp 89-101 summarises the various types of materials used. These range from disperse polymers in water such as those used in latex paints to fully water soluble solution polymers. A further class of binders are referred to as water reducible resins which are a hybrid between the water insoluble polymer binders and fully water soluble binders. Such hybrid resin systems usually require significant levels of organic cosolvents to be present and these systems are intended for industrial rather than domestic household applications. An example of such a resin system is disclosed in U.S. Pat. No. 4,230,609 (Burroway et al).
The use of copolymerisable acrylic acid in addition copolymers is summarised in Table 2 of the Padget article and this shows the general effect of increasing the acrylic acid level from 0 to 100%. The viscosity characteristics on neutralisation are set out as the level of acid is increased. At acrylic acid levels of 1% or more the viscosity is described as increasing on neutralisation. At levels of acrylic acid of from 1-2% this viscosity increase is described as being substantially overcome by using a hydrophobic co-monomer such as styrene. This would be as a replacement for a less hydrophobic hard co-monomer such as methyl methacrylate. We have found that while maintaining viscosity stability such latexes tend, when formulated as paints, to perform unsatisfactorily for tint strength and stain resistance. We have observed similar performance in paints formulated from latexes described in Australian Patent No. 500,903 in the name of Hoechst. Example 1 of that patent describes the preparation of a butyl acrylate/styrene/methacrylic acid/acrylic acid/acetoacetic acid allyl ester in the weight proportions of 300/300/12/6/12. This calculates as 1.90% methacrylic acid and 0.95% acrylic acid on total monomers.
In Rohm and Haas European Patent No. EP 0 466 409 the preparation of a number of latexes is described for use in blending hard and soft latexes to provide paints with good block resistance. The composition of the samples containing carboxylic acid co-monomers are set out in TABLE 1 where the monomer amounts are by weight:
TABLE 1Sample NoEHABAMMAStyreneANMAA1—110539.1496.4—42.52—45919.51179—343—969680——344—4591190——345510—393695.3425346855.1—39.3350.2425347—96939.3640.9—348—81639.3793.9—34EHA = Ethyl hexyl acrylateBA = Butyl acrylateMMA = Methyl methacrylateAN = AcrylonitrileMA = Methacrylic acid
In samples 2-8 the methacrylic acid level is approximately 2.0% and for sample 1 it is 2.5%.
PCT Patent Application No. PCT/AU94/00600 discloses water-borne soil resistance coatings. These coatings-make use of a binder that is a blend of low and high Tg polymeric dispersions in particular ratios. While soil resistant coatings of this invention have good properties with regard to soil resistance they do not exhibit very good stain resistance properties when tested as interior trim paints.
Aqueous paints which comprise latex polymer particles having different hardnesses have been proposed to improve specific properties of paint films including film hardness, block resistance, coalescence and soil resistance.
In Rohm and Haas European Patent No. EP 0 466 409, referred to previously, blends of hard and soft emulsion polymers are used. The hard emulsion polymer has a Tg greater than 20° C. and the soft emulsion polymer has a Tg less than 20° C. The compositions described in this prior art specification are claimed to be particularly useful in providing good block resistance when formulated as interior semi-gloss paints whilst retaining good film formation properties at low temperature. No statement is made about stain resistance. At page 5 of this prior art specification the inventors explain that if a soft emulsion polymer is blended with increasing amounts of a hard emulsion polymer, the minimum film forming temperature (MFFT) of the blend will remain about equal to the MFFT of the soft polymer until the hard polymer is about 50 weight percent of the polymer blend. As it is important for their compositions to have good low temperature film coalescence the preferred binder compositions have 60% by weight soft and 40% by weight hard polymer. In their examples the soft to hard latex polymer ratio varies from 4:1 to 1.5:1 and all the soft polymers have a MFFT greater than 0° C.
Japanese patent J 5 9215-365-A in the name of Nippon Acryl Kagaku discloses compositions obtained by mixing 35-45 wt % acrylic resin emulsion of Tg 25 to 50° C. and 55-65 wt % acrylic resin emulsion of Tg −10 to +15° C. These compositions thus have a soft to hard ratio of 1.9:1 to 1.2:1. These compositions also have an organic plasticiser which may be volatile. Whilst these paints have particular application as ventilator coatings to prevent the smearing of the surface, stain resistant properties are not examined, though they would be expected to be poor based on the monomer composition.
Hoy discusses the effect of a number of composition and processing variables in an article entitled “Effect of Reaction Pathway on Emulsion Polymer Structure” in Journal of Coatings Technology Vol 51, No. 651 April 1979 at pages 28-41. In a number of model systems he examines the effect of high Tg monomer on viscosity in very dilute systems. His model systems use various monomers and when he uses styrene his study is confined to polymeric dispersions where the overall Tg of the dispersions was from 27 to 100° C. His study suggests that the use of styrene as a co-monomer would lead to polymeric dispersions where the carboxylic groups were buried.
Australian patent No. 666 797 discloses latex paints having good stain resistance properties. These paints are required to have a non-cellulosic thickener and use a binder that has at least 40 mole percent of styrene and α-methyl styrene. An example of a preferred polymer is a copolymer of 49 mole % styrene, 11 mole % α-methyl styrene, 22 mole % 2-ethyl hexyl acrylate and 18 mole % methyl methacrylate. This preferred composition together with other preferred latex compositions do not have present any carboxylic acid moieties. While we have found paints described in this patent have generally good stain properties they are less than optimum for stain resistance to certain common household staining materials.